Surely the best way would be to let the child decide.

I'm left handed but play right handed and with a guitar held this way a lefty definately has an advantage. BUT some lefties are not totally left handed in all cercumstances for instance i write with my left hand, practise archery left handed but hold a cricket bat, kife & fork
right handed.
In the UK they used to force left handed children to learn to write right handed and caused much distress, thank goodness that went out of favour many years ago!
So after rambling on, i mean to say that the child will naturally pick the instrument up and find the most comfortable way for them to play it.
It's hard enough to get them interested in the first place so don't make it even more difficult for them.